Hypercapnia, elevation of PCO2 in blood and tissue, commonly occurs in severe acute and chronic lung disorders, such as chronic obstructive pulmonary disease (COPD). Patients with advanced COPD frequently develop bacterial lung infections, and hypercapnia is a risk factor for mortality in such individuals, as in those with community-acquired pneumonia. We have shown that hypercapnia suppresses transcription of innate immune response genes required for host defense in human, mouse and Drosophila cells, and increases mortality due to bacterial infections in mice and Drosophila. These findings suggest that hypercapnia is not simply a marker of advanced lung disease, but plays a causal role in poor clinical outcomes by increasing susceptibility to infection. Our data also strongly suggest that hypercapnia inhibits innate immunity and host defense by pathway(s) conserved from Drosophila to mammals. Because the molecular mediator(s) of hypercapnic immune suppression are undefined, we conducted a genome-wide RNAi screen in cultured Drosophila cells, which identified ~140 genes required for hypercapnic suppression of antimicrobial peptide (AMP) genes. The 5 most potent of these candidate CO2-mediators encode the zinc finger homeodomain transcription factor, Zfh2; a histone deactylase; a histone methyltransferase; a chromatin-associated Ig-repeat protein; and a Rac-interacting protein. None of these genes were previously known to have immunoregulatory function. Exciting new data indicate that mutations in zfh2 protect adult Drosophila against CO2-induced host defense defects. Thus, we hypothesize that Zfh2 and the proteins encoded by the other 4 candidate genes are components of pathway(s) by which hypercapnia suppresses innate immune gene expression and host defense in Drosophila, and that the mammalian orthologs of these genes mediate hypercapnic suppression of innate immune/host defense genes in mouse and human phagocytes. In the case of Zfh2 and its mammalian orthologs, ZFHX3 and ZFHX4, we further hypothesize that hypercapnia alters their abilities to bind target gene promoters or components of the NF-:B transcriptional complex, or modifies the transcriptional activity of these factors, thereby decreasing expression of specific innate immune/host defense genes. In the proposed studies, we will test these hypotheses in vitro using cultured fly, mouse and human macrophages, and in vivo using adult Drosophila and lung inflammatory cells obtained from mice with Pseudomonas aeruginosa pneumonia. These studies will for the first time define components of conserved pathway(s) by which hypercapnia impairs innate immunity and host defense, and determine their mechanisms of action. The results should lay the basis for future studies aimed at preventing hypercapnic immune suppression in patients with advanced lung disease.